The present invention relates to direct current (dc) motors and magnetic clutches. More particularly, the present invention relates to methods and apparatus for improving the efficiency of permanent magnet brushless dc motors and magnetic clutches.
Permanent magnet brushless dc motors and magnetic clutches are known in the art. To facilitate discussion, FIG. 1 is a diagrammatic sectional view of a spindle motor 100, representing a typical permanent magnet brushless dc motor. The spindle motor is selected herein for discussion to facilitate ease of understanding although, as will be seen, the inventive concepts disclosed herein apply equally well to other types of direct current motors (e.g., other types of radial gap dc motors, axial gap dc motors, external stator dc motors, and the like), and magnetic clutches. Spindle motor 100 includes a rotor portion 102 that is rotatable about a stationary stator portion 104. Rotor portion 102 has a multi-pole permanent magnet 106 mounted to a back yoke 108 and is typically formed of a continuous ring of a suitable magnet material. Magnet 106 is typically magnetized in segments, with adjacent segments being alternately magnetized in opposite directions. Back yoke 108 serves as a return path and may be formed of soft iron or steel.
Stator portion 104 is separated from rotor portion 102 by a generally cylindrical air gap 110. Stator portion 104 includes a stator yoke 112 having stator teeth 114, typically formed of laminated sheets of soft iron or silicon steel, with stator teeth 114 oriented toward rotor portion 102 and carrying excitation coils 116. Stator portion 104 is typically supported on a base plate support structure (omitted from FIG. 1 to improve clarity).
As is well known to those skilled in the art, rotor portion 102 is induced to rotate about stator portion 104 when excitation coils 120 are electrically energized. Although all dc motors convert electrical energy in the form of current through their excitation coils into the rotor's rotational force, they do so at various levels of efficiency. In general, a dc motor having a high level of efficiency is desirable over a less efficient dc motor. High efficiency motors tend to generate less heat and noise during operation and usually operate for a longer period of time for a given battery charge. These considerations are particularly important for manufacturers of computer disk drives, as these devices are often employed heat and/or noise sensitive environments, or in battery-operated portable computers.
In the design of dc motors, it is known that the magnetization pattern in the dc motor magnet, e.g., magnet 106 of FIG. 1, has a strong effect on motor efficiency. Designers of dc motors constantly search for patterns that, all things being equal, yield the highest level of efficiency. Once a suitable pattern is selected, it may then be furnished to a magnetizing fixture designer who, using conventional techniques, designs a magnetizing fixture to reproduce as closely as possible in the motor magnet the desired magnetization pattern.
It is widely believed that the most efficient magnetization pattern in the dc motor magnet is a purely radial pattern. To facilitate further discussion, FIG. 2A illustrates a portion of spindle motor 100 of FIG. 1, including permanent magnet 106 having a purely radial magnetization pattern. Within permanent magnet 106, adjacent magnet segments are alternately magnetized in opposite directions. This is illustrated in the magnetization lines of magnet segments 202, 204, and 206 (depicted in FIG. 2A as series of parallel arrows in cylindrical permanent magnet 106).
In the purely radial magnetization pattern of FIG. 2A, the magnetization lines are radial throughout permanent magnet 106, including magnetization lines that are adjacent to the segment boundaries. Across a segment boundary, i.e., the imaginary line that distinguishes a magnet segment from its adjacent neighbor, magnetization lines of a purely radial magnetization pattern are oriented in opposite directions right next to one another. This condition is illustrated in FIG. 2A at boundaries 208 and 210, which separate respective adjacent magnet segment pairs 202/204 and 204/206. It should be noted that although the purely radial pattern of FIG. 2A has long been held by many to be the most efficient, it is difficult to manufacture economically.
It is also known that dc motors employing the purely radial magnetization pattern of FIG. 2A would suffer from high cogging forces. Cogging forces represent a well known phenomenon, and are generally a result of preferred angular position that the rotor has due to the magnetics. A high level of cogging forces adversely affects motor performance, particularly in the ability of the rotor to smoothly rotate about the stator. Excessive cogging forces may also lead to an unacceptably high level of acoustic noise. Consequently, while manufacturers strive to achieve the highest efficiency level believed to be associated with the purely radial magnetization pattern, other alternative magnetization patterns have been developed to achieve an acceptable tradeoff between high efficiency and high cogging forces and/or high manufacturing costs.
With these alternative magnetization patterns, however, it has been found that motor efficiency is reduced. FIG. 2B illustrates one such alternative magnetization pattern, wherein the boundaries between adjacent magnet segments 224/226 and 226/228 are intentionally expanded into unmagnetized transition regions 230 and 232. The unmagnetized transition regions between adjacent magnet segments are known as dead zones since no magnetization lines exist in these unmagnetized transition regions. In the regions that are magnetized, e.g., within magnet segments 224, 226, and 228 of FIG. 2B, the magnetization lines are kept substantially radial. However, the presence of the unmagnetized transition regions, or dead zones, robs the dc motor of a portion of its driving force, resulting in a concomitant reduction in the level of motor efficiency.
Another approach involves sinusoidal magnetization lines, with the orientation of a given magnetization line being determined by its angular position from the boundary between adjacent magnet segments. In accordance with this sinusoidal magnetization scheme, each magnetization line has a radial component which varies from about 90.degree. (almost purely radial) at the center of a magnet segment to almost 0.degree. at the segment boundary. Such an approach is described in, for example, U.S. Pat. No. 5,418,414, issued to Ackermann, et al. However, the efficiency of the motor that results has been found to be lower than the efficiency level of an analogous dc motor employing the purely radial magnetization pattern.
In view of the foregoing, improved methods and apparatus for magnetizing permanent magnets in dc motors or magnetic clutches are desired. The improved methods and apparatus preferably yield a dc motor or a magnetic clutch that can operate with a high level of efficiency.